EXPERIMENTS AT CERN IN THE DECADE 1964 – 1974 Luigi Di Lella Physics Department, University of Pisa (retired CERN physicist) 6th June 2014
. The CERN physics “environment” in the years 1964 – 74 Size of experiments, detectors, data acquisition, analysis . A personal selection of experiments at the PS and ISR
For a more complete description of physics results from those years see: L. Van Hove and M. Jacob, Highlights of 25 years of physics at CERN, Physics Reports 62 (1980) 1
The CERN physics environment in the years 1964 – 74
. Many experiments (typically, ~ 10 / year) with relatively short data – taking time (typically, one month) . Small groups ( < 10 physicists) . Little theoretical guidance (no Standard Model yet !) . Two classes of experiments (managed by two CERN Divisions): – Bubble chamber experiments – Electronic experiments
Bubble chambers (invented by D.A. Glaser in 1952) Volumes filled with liquid close to the boiling point, kept under pressure. Few milliseconds before the arrival of the beam particles (fast ejection) expansion by a piston produces a pressure drop superheated liquid bubbles form first on the ionization providing photographic images of charged particle tracks
Used liquids: H2, D2, He4, Ne, C3H8 (propane), freon, Xe
No trigger on selected events ; Rate limitation from number of tracks / picture .
Example of crowded picture
Event photographs (stereo views) are scanned by physicists and track points are digitized (manually, then automatically) and stored onto magnetic tapes Charged particle tracking in electronic detectors is achieved by spark chambers (invented by Fukui and Miyamoto in 1959)
High voltage pulse applied between these two lines following an external trigger
Sparks occur along a charged particle track when the high voltage pulse is applied (not later than 0.5 ms after the track time) Max. trigger rate ~ few per second Metallic plates in a volume filled with pure noble gases (typically Ne-He mixtures)
Initially, spark chambers are “read out” by photographic cameras providing stereo views of events. Event photographs are then scanned and digitized as for bubble chambers . Data acquisition in electronic experiments
On – line computers appear around 1964 and are used more and more for data acquisition and detector monitoring . They allow the development of more “automatic” spart chamber read −out techniques: . Sonic chambers (with microphones in the corners of each plane to detect the spark noise and measure its position from signal timing) . Magnetic core read-out: the ground planes are made of wires traversing ferrite cores which are magnetized by the spark current and read out as a computer memory (made also of ferrite cores in those times) . Magnetostrictive read-out: detection of the magnetic perturbation induced by the sparks on a nickel ribbon perpendicular to the ground wires, and propagating along the ribbon with the speed of sound.
Between the late 1960s and the early 1970s, spark chambers are gradually replaced by Multi−Wire Proportional Chambers and Drift Chambers (invented by G. Charpak in 1968) Data analysis
Magnetic tapes containing the digitized information from tracking detectors (bubble chambers, spark chambers, MWPCs) are analysed in the CERN Computer Center or in external Institutes using reconstruction and analysis programs generally written in Fortran During data – taking a few tapes are taken to the CERN Computer Center by one of the physicists on shift for fast analysis of a subsample of events to monitor the detector performance (“Bicycle on − line”) A personal selection of experiments at the PS and ISR
. Neutrino experiments . Experiments on CP violation . Hadron spectroscopy . Exclusive reactions at high energy . First results from ISR experiments . Searches for fractionally charged particles . The three muon “g – 2” experiments Neutrino physics at CERN begins in the PS South Hall in 1963 Layout of the beam and experiments
Two innovations with respect to the 1962 experiment nd at the Brookhaven AGS which discovered the 2 neutrino (nm ) : . Fast extraction of the primary proton beam from the PS and use of an external target to produce pions and kaons decaying to neutrinos; . The invention of the magnetic horn Wide band neutrino beams Charged hadrons (p± , K±) produced at the target are focussed into an almost almost parallel beam with wide momentum distribution by magnetic “horns” (invented at CERN by Simon van der Meer)
. Axially symmetric conductors . Pulsed current . Magnetic field normal to hadron momentum
Change current polarity select opposite charge hadrons + - p ( → nm ) p ( → nm ) Simon van der Meer explaining the horn working principles Horn Horn inner conductor outer conductor
Horn installed on the beam line Typical neutrino energy spectrum (1964) (wide-band, horn-focused beam from the CERN PS) Neutrino detectors
1. A Heavy Liquid Bubble Chamber (HLBC) filled with CF3Br (freon) in a 2.7 T magnetic field Diameter 1.2 m, volume 500 liters (0.75 tons of freon) Nuclear interaction mean free path in freon ~ 0.6 m , radiation length 11 cm
2. A spark chamber detector with a fiducial mass of ~15 tons
beam
Thick Fe + Pb plates Thin Al plate + spark chambers spark chambers + 4 scintillation counters Magnetic field (1.8 T) + thin plate spark chambers Spark chamber picture of a - nm + n m + p “quasi-elastic” event 418 events in the 1963-64 runs
Spark chamber picture of a - ne + n e + p “quasi-elastic” event 39 events in the 1963-64 runs
(consistent with expected ne flux)
1966 – 74 : a new neutrino beam in the PS South – East Area
DETECTORS
1966 – 70 : The 1.2 m diam. Heavy Liquid Bubble Chamber filled with C3H8 (propane) followed by a spark chamber detectors; 1971 – 74: The giant Heavy Liquid Bubble Chamber Gargamelle filled with freon Among the results from the early neutrino experiments: . Measurement of the cross-section versus energy of the quasi-elastic process - nm + n m + p . Measurement of the nucleon weak axial form factor from the Q2 distribution of quasi-elastic events . Measurement of the cross-section versus energy of resonance production: - ++ - + nm + p m + D m + p + p - + - + nm + n m + D m + n + p - + - nm + n m + D m + p + p° . Study of multi-pion production by neutrinos . Search for m-e+ and m-m+ pairs as a possible signature for the production of a light (mass < 2 GeV) weak boson W: n + nucleus m- + nucleus + W+ , followed by W+ e+ + n or W+ m+ + n m Measurement of the neutrino – nucleon total cross-section in the 1.2 m diameter heavy liquid bubble chamber I. Budagov et al., Phys. Lett. 30B (1969) 364
-38 2 sTOT(nm– nucleon) = (0.8±0.2) E (GeV) x 10 cm
The linear rise with energy is a consequence of the quark structure of the nucleon, which had just been discovered at SLAC by measuring “deep-inelastic” electron – nucleon scattering Demonstration that neutrinos interact with light, point-like nucleon constituents See the recent paper by D.H. Perkins “An early neutrino experiment: how we missed quark substructure in 1963”, The European Physics Journal H 38 (2013) 713 Why did it take ~10 years to discover neutrino Neutral – Current interactions ? -39 2 n + nucleon → n + hadrons [cross–section typically ≈ 2 x 10 En cm (En in GeV)]: Incident neutrino energies 1 – 2 GeV little visible energy in the detector difficult to separate the interaction from interactions of neutrons produced by neutrino interactions near the end of the shielding wall muon decay tunnel
target nm protons horn detector shielding Thicker shielding does not help to reduce this background neutron interacting In spark chamber detectors, for events with no muon in detector the trigger is inefficient to low energy hadronic showers Attempt to detect the elastic reaction n + p n + p in the 1.2 m diam. HLBC filled with propane (D.C. Cundy et al, Phys. Lett. 31B (1970) 478) Observe 4 events with proton kinetic energy 150 – 500 MeV corresponding to
s(nm + p nm + p) - 0.12 0.06 (the authors consider this result as an upper limit). s(nm + n m + p)
In 1979 this ratio is measured to be 0.11 ± 0.02 by the BNL-Harvard-Pennsylvania coll. 30 ton liquid scintillator contained in 216 independent cells 217 events (background 38%) See A. Entenberg et al. Phys. Rev. Lett. 42 (1979) 1198 nm ( nm ) + electron → nm ( nm ) + electron -42 2 Very small cross–section, typically = A x 10 En cm (En in GeV) ; Background from ne – electron scattering (Charged – Current interaction)
n e– n , n e 2 The factor A depends on sin qW ne ne Z W + W - (unknown until 1973) Present values: – – e e n : A 9.5 ; n : A 3.4 e e– n e e e ne only all three n n m : A 1.6 ; n m : A 1.3 neutrino e only flavors
The general opinion in the early 1970s: if neutrino Neutral – Current interactions exist at all, one needs neutrino beams from a higher energy proton accelerators to discover them: . Higher cross-section, higher visible energy ;
. Longer muon tracks from nm Charged – Current interactions easier separation between the two interaction types Gargamelle . Designed and built in France by André Lagarrigue and collaborators . Cylindrical body 4.8 m long, 1.85 m diameter, volume 12 m3 . Magnetic field 1.8 T
André Lagarrigue
Gargamelle during assembly
Inside the chamber body 1973: observation of an event consisting only of an electron collinear with the beam in the heavy liquid bubble chamber Gargamelle from an exposure to the
antineutrino beam ( mostly nm ) Electron energy 385 ± 100 MeV ; electron angle to beam direction 1.4o ± 1.4o F.J. Hasert, et al., Phys. Lett. 46B (1973) 121
Fluxn e 1% Fluxn m
Antineutrino beam direction
- - o Expected number of ne + e → ne + e events with Ee > 300 MeV, qe < 5 : 0.03 ± 0.02 Observation of neutrino-like interactions without muon or electron in Gargamelle F.J. Hasert, et al., Phys. Lett. 46B (1973) 138 Events with neutrino beam: 102 Neutral-Current (NC), 428 Charged-Current (CC) events Events with antineutrino beam: 64 NC, 148 CC events Study also Associated Stars (AS): neutron stars associated with a CC event giving a muon visible in the chamber. Require total visible energy > 1 GeV in NC events, total hadronic energy > 1 GeV in CC events.
Distributions of event origin along the beam axis: NC and CC distributions are similar, consistent with uniform distributions as expected for neutrino interactions .
(NC/CC)n 0.210.03
(NC/CC)n 0.450.09
Associated Stars show decreasing distributions, as expected from the known neutron interaction length. Example of a neutrino interaction producing a hadronic shower and no muon
Neutrino beam direction Demonstration that quarks have fractional electric charge Compare deep-inelastic electron scattering with n + n Charged Current scattering from a target containing equal numbers of protons and neutrons ( = equal numbers of up- and down- quarks and antiquarks). 2 Form of the structure function F2(x,y) in the quark model (x = Q /2MpEhad , y = Ehad /E) : . for electron scattering em 2 2 2 5 2 2 2 1 4 1 F (x, y) [1+ (1- y) ]x[ e q(x)+ e q(x)] [1+ (1- y) ]x[q(x) + q(x)] eq eq + 2 q q 18 2 9 9
. for n + n scattering 18 Fn+n (x, y) [1+(1- y)2]x[q(x)+q(x)] Fn+n Fem 3.6Fem 2 2 5 2 2
Points with error bars: Gargamelle data H. Deden et al, Nucl. Phys. B85 (1975) 269
CERN experiments on CP violation The “July 1964 revolution”: evidence for p+p- decay of the long – lived K0 meson in an experiment at the Brookhaven AGS – a decay violating CP symmetry Decay rate ~ 2 x 10-3 with respect to all charged decay modes J.H. Christenson, J.W. Cronin, V.L. Fitch, and R. Turlay, Phys. Rev Lett. 13 (1964) 138 The two mass eigenstates of the neutral K mesons before July 1964:
K0 + K0 K0 - K0 K0 K0 1 2 2 2 short–lived, CP = +1 long–lived, CP = –1
After July 1964:
0 0 0 0 0 0 0 0 K 1 + εK 2 (1+ ε)K + (1- ε) K K 2 + εK 1 (1+ ε)K - (1- ε) K KS K L 1+ ε 2 2(1+ ε 2 ) 1+ ε 2 2(1+ ε 2 )
short–lived long–lived An early suggestion to explain the violation of CP symmetry + - in KL p p decay : (Bell & Perring, Bernstein, Cabibbo and Lee) the existence of a new long – range weak vector field producing a potential energy of equal magnitude but opposite sign for K0 and K0 . A K0 – K0 mass splitting of ~10-8 eV could produce + - KL p p decays at the rate measured in the AGS experiment but the rate would vary with the square of the KL energy Repeat the AGS experiment at a much higher beam momentum
(the average KL momentum in the AGS experiment was 1.1 GeV/c) + - Measurement of KL p p at a mean KL momentum of 10.7 GeV CERN – Orsay – MPI collaboration, Phys. Lett. 15 (1965) 58
. Neutral beam derived from an internal target at the PS . Beam angle 8 mrad with respect to the circulating proton beam . Charged pion / electron / muon separation by gas Čerenkov counter + Fe absorber . Magnetic spectrometer: dipole magnet + optical spark chambers
Results: + - . 44 ± 8 events consistent with KL p p decay . Rate (3.5 ± 1.4 ) x 10-3 with respect to all charged modes, consistent with the AGS result Another early suggestion to explain the apparent violation of CP symmetry + - in the decay KL p p : violation of C symmetry in the electromagnetic interaction of hadrons, inducing violation of CP symmetry in weak hadronic decays through higher order corrections (Lee & Wolfenstein, Okun’, Prentki & Veltman)
1966: Experimental evidence for charge asymmetry in the decay h p+ p- p° (an electromagnetic process) C. Baltay et al., Phys. Rev. Lett. 16 (1966) 1224 Bubble chamber experiment at the Brookhaven AGS studying the reaction p + d h + p + p followed by h p+ p- p° ( 1441 events ) + - In the h rest frame define N+ : number of events with T(p ) > T(p ) - + N- : number of events with T(p ) > T(p )
N+ – N - Measured asymmetry A = = 0.072 ± 0.028 N+ + N - The CERN – ETH – Saclay experiment at the CERN PS A.M. Cnops et al., Phys. Lett. 22 (1966) 546 - p + p h + n ; beam momentum 713 MeV/c ; 12 cm long liquid H2 target Top view Dipole magnet: B = 0.72 T
neutron counters
Magnetic compensator 36 gap thin foil View from target spark chamber Identify p- + p h + n by neutron time-of-flight and angle (missing mass) Frequent magnetic field reversals to reduce spurious charge asymmetries Result based on 10665 h p+ p- p° decays :
N+ – N - Measured asymmetry A = = 0.003 ± 0.011 N+ + N - no evidence for C violation in h p+ p- p° decay + - Precise measurements of the KL p p decay parameters + - A(K L p p ) h+- h+- exp(i+- ) + - (ratio of decay amplitudes); Dm mL -mS A(KS p p ) Two basic methods: + - + - . Measure the interference between KL p p and Ks p p from coherent regeneration of Ks by one or more regenerators . Interference term behind one regenerator : 1 - ( + )t 2 S L 2h+-e cos(Dmt + -+- ) i e : KS coherent regeneration amplitude relative to KL ; t : time in the KS,L rest frame measured from regenerator exit + - + - . Measure the interference between KL p p and Ks p p from initial K0 or K0 states (“vacuum regeneration”). Interference term : 1 - ( + )t 2A(p)h e 2 S L cos(Dmt - ) +- +- S(p) - S(p) A ( p ) ( S(p), S(p) : initial K0 and K0 production intensities); S(p) + S(p) 0 0 t : time in the KS,L rest frame measured from the K ( K ) production point
+ - + - Measurement of the interference between KL p p and KS p p behind a copper regenerator H. Faissner et al. (Aachen-CERN-Torno coll.), Phys. Lett. 30B (1969) 204
. Magnetic spectrometer with magnetic core read-out spark chambers . Trigger counters select events with two charged particles parallel to the beam axis + - after momentum deflection high efficiency for KL ,S p p decays . Five copper regenerators of equal thickness but different densities + - KS p p (regenerated KS)
1 - ( + )t 2 S L Interference term 2h+-e cos(Dmt + -+- ) + - KL p p
Regeneration amplitude = f(0) – f(0) : difference between the K0 and K0 forward scattering amplitudes (from other experiments)
Time in the K rest frame measured from the regenerator exit (units of 10–10 s) Precision measurement of Dm with two regenerators Aachen –CERN-Torino coll., Phys. Lett. 32B (1970) 523
. KL beam subdivided into three parallel regions with different regenerator configurations
. Equal KL attenuations for the three regions DN(t) N12(t) - N1(t) - N2 (t) + N0 (t) 1 2 cos(Dmt)
Nb. events no regenerator = 0 at t = p / 2Dm
Dm = (0.542 ± 0.006) x 1010 s−1 The CERN “vacuum regeneration” experiment (CERN – Heidelberg collaboration)
Side view
Top view
. The first CERN experiment to use large-size multiwire proportional chambers on a large scale for track reconstruction and trigger ~ 103 recorded events / machine cycle (0.35 s) . Front face of the decay volume 2 meters from K0, K0 production target Time distribution of K p+p- decays in the “vacuum regeneration” experiment CERN – Heidelberg coll., Phys. Lett. B48 (1974) 487
Results : −3 |h+-| = (2.300 ± 0.035) x 10 10 −1 S = (1.119 ± 0.006) x 10 s 0 0 f+- = (49.4 ± 1.0) +[(Dm – 0.540)/0.540] x 305 Measurement of Dm from the charge asymmetry in semileptonic K decays CERN-Dortmund-Heidelberg coll., Phys. Lett. B52 (1974) 113 Lepton charge asymmetry in the “vacuum regeneration” experiment d(t) = (N+ − N −)/(N+ + N −) vs. decay time in the K rest frame
p m ν p e ν 0.02 0.02
10 x 10–10 s
10 x 10–10 s
1 - ( + )t δ(t) 2A( p)e 2 S L cos(Dmt) + 2Re ε
(For KL semileptonic decay d = 2Re e ≈ 0.3 % as a consequence of the DS = DQ rule) Dm = (0.5334 ± 0.0040) x 1010 s−1 The parameter e describes CP violation in K0 – K0 mixing
(the small CP = +1 impurity in the KL state) Is there also direct CP violation in the weak decay matrix elements? Denoting by e’ the parameter describing direct CP violation the ratios of decay amplitudes are + - A(KL p p ) h+- + - e + e' A(KS p p )
0 0 A(KL p p ) h00 0 0 e - 2e' A(KS p p )
0 0 motivation to search for KL p p and measure its rate
Early experiments with 10 – 20 events gave contradictory results. 0 0 Measurement of the decay KL p p Aachen – CERN – Torino coll., Phys. Lett. B40 (1972) 141
View along beam Side view Events with 4 detected photons (at least 2 converting in the spark chambers)
0 0 K p0p0 KL p p S (regenerated K ) h S Final result: 00 = 1.00 ± 0.06 h+− First evidence for e’ ≠ 0 from experiment NA31 at the CERN SPS (1988) Conclusive result in 1999 from KTEV (Fermilab) and NA48 (CERN): e’ / e = (1.66 ± 0.23) x 10−3 Hadron spectroscopy Many important results, mainly from the hydrogen bubble chambers: . Measurement of the K*(890) spin – parity (1– ) from p p annihilation at rest . Discovery of the f (1270) and measurement of its spin – parity (2+) + . Discovery of the A2 (1320) and measurement of its spin – parity (2 ) . Discovery of the K*(1430) and measurement of its spin-parity (2+)
K− + p K0 + p− + p p+ + p− + p− + p 3.5 GeV/c K− beam in the Saclay 80 cm hydrogen bubble chamber ( K0 p− ) invariant mass distribution
These results provided additional evidence for SU(3) multiplets of hadronic resonances, which led to the formulation of the hadron quark model Exclusive reactions at high energy Measurement of the charge exchange reaction p− + p p0 + n Orsay – Saclay coll., Phys. Rev. Lett. 14 (1965) 763 ; Phys. Lett. 20 (1966) 75
Anticoincidences Photon detector Spark chamber with thin Pb plates Differential cross – section ds/d|t|
2 t (P - - P 0 ) p p
. Minimum at -t ≈ 0.6 (GeV/c)2 . Decrease of ds/d|t| with increasing energy . The decrease with energy becomes larger with increasing |t| (“shrinking” of the forward peak) All features quantitatively described by the Regge pole model (exchange of the – meson Regge trajectory) Other predictions of the Regge pole model:
. Polarization parameter in pion – proton elastic scattering P0 ≠ 0 . Opposite sign for p+p and p−p elastic scattering − 0 . P0 = 0 for p + p p + n Differential cross – section for a polarized proton target: ds ds (1+ P P ) d | t | d | t | 0 T PT 0
PT : polarization of target protons ; P0 direction normal to the scattering plane
Crystals containing ~1/16 free, polarized protons with >70% polarization had been developed in Saclay in the early 1960s by A. Abragam and collaborators, and brought to CERN in 1964. Polarization reversal was achieved by changing a microwave frequency by ~0.3 %, with no need to reverse the magnetic field no systematic effects
The CERN – Orsay scattering experiment M. Borghini et al., Phys. Lett. 21 (1966) 114 ; Phys. Lett. 24B (1967) 77 Select scattering on free, polarized protons (1/16) by two-body kinematics
(pion – proton coplanarity and qp – qp correlation)
1 N(PT up) - N(PT down) Polarization parameter : P0 PT N(PT up) + N(PT down)
A simple Regge pole model does not describe correctly the polarization parameter in BOTH p+p and p−p elastic scattering Polarization parameter in p– + p p0 + n Orsay – Saclay – Pisa coll., Phys. Lett. 23 (1966) 501
Scintillation counters for neutron detection
Polarized target
The simple Regge pole model with the exchange of only one trajectory (the – meson) predicted P0 = 0 in disagreement with the experimental results. It was still possible to describe the polarization results for both p± p elastic scattering and p– p charge exchange using Regge pole models with more trajectories and more fitting parameters. However, by the mid 1970s physicists lost interest in these studies, probably attracted by new, more interesting subjects The CERN Intersecting Storage Rings (ISR) The first proton – proton collider ever built . Two slightly distorted rings intersecting in 8 points . Average radius 78.6 m . Proton accumulation by RF stacking in momentum space: the first proton pulse from the PS is accelerated by the ISR RF system up to the highest acceptable momentum (the orbit with largest radius), successive pulses are accumulated on orbits of lower and lower average radius . Circulating beams are ribbon – shaped (few cm wide), crossing at a 14° angle, with no time structure . Max. proton energy 31 GeV collision energy 62 GeV corresponding to ~ 2.05 TeV protons on a stationary target . Design vacuum 10−10 Torr, soon improved to 2 x 10 −12 . Design luminosity 4 x1030 cm −2 s −1 reaching > 2 x1031 with low-b insertions . First collisions 27 January 1971; end of collider operation 1984 View of an ISR crossing region with a double – arm detector at 90 degrees 1964 : Presentation of the ISR design report to CERN Council End of 1965: approval (“a window to investigate the highest energies”) 1968: Study groups to prepare for experiments (Which physics should be studied at the ISR?) Emphasis on: measurement of proton – proton total cross – section; elastic scattering including Coulomb interference region; isobar production p + p p + N*; particle production.
Cosmic ray experiments had shown that the main feature of pion production at very high energies is the limited transverse momentum pT
dN -6pT p e (pT in GeV/c) T dp T giving
+B −B
Split Field Magnet with magnetic field directions The story of an ISR discovery which prevented a more important discovery 1970: a “beam dump” experiment at the Brookhaven AGS observing the production of high – mass muon pairs J.H. Christenson, G.S. Hicks, L.M. Lederman. P.J. Limon, B.G. Pope and E. Zavattini, Phys. Rev. Lett. 25 (1970) 1523
J/y m+m- ?
Measurement of the muon angle by counter hodoscopes and of muon energy by residual range in iron poor resolution on the dimuon invariant mass Top view (both arms)
Side view of one arm p + p e+e− + X Detect electrons near 90° at opposite azimuth Good invariant mass resolution (wire spark chambers + lead glass counters) The preliminary proposal had gas Čerenkov counters for electron / pion separation.
The final set-up had no gas Čerenkov counters (only low pT pions were expected around 90°), but the solid angle was much increased (about 1 sr on each side of the beams)
ISR experiment R-103 Final set-up CERN– Columbia – Rockefeller coll., Phys. Lett. B46 (1973) 471 Results presented at the 1972 Int. Conf. on High Energy Physics
R-103
p0 transverse momentum (GeV/c) The production of high transverse momentum p± was also observed at the ISR in two experiments with single-arm magnetic spectrometer
British-Scandinavian collaboration (ISR experiment R-203), Phys. Lett. B44 (1973) 521 R-102 Saclay-Strasbourg collaboration (ISR experiment R-102), Phys. Lett. B44 (1973) 537 The production of high transverse momentum hadrons at the ISR was interpreted as the result of hard scattering between point – like proton constituents (“partons”, first observed in deep – inelastic electron – nucleon scattering at SLAC). It had been predicted to occur in high-energy proton-proton collisions: Inclusive Processes at High Transverse Momentum, S.M. Berman, J.D. Bjorken and J.B. Kogut, Phys. Rev. D4 (1971) 3388.
However, the production cross-section of high pT hadrons calculated in this paper assumed photon exchange between partons (electromagnetic interaction), too small to be observed at the ISR. The ISR experiments have demonstrated that partons behave as point-like objects also when they interact strongly. These results have opened the way to the study of jet production in high energy hadron collisions (the only process occurring to leading order in perturbative QCD). In experiment R-103 (the large solid angle double-arm lead-glass array) the rate of two-arm coincidences used for trigger was dominated by the production of high pT p0 pairs emitted at opposite azimuth . It was limited to ~10 Hz by the spark chambers (and also by the rate of event writing onto magnetic tape). The only way to keep the trigger rate under control was to increase the trigger energy threshold.
ISR experiment R-105 ( two-arm spectrometer ) CERN-Columbia-Rockefeller-Saclay collaboration
e+e− invariant mass distribution Phys. Lett. B56 (1975) 482 A clear J/Y e+e− few months after its discovery at BNL and SLAC The rise of the proton – proton total cross – section at the ISR
Three experiments: R – 801 (Pisa – Stony Brook collaboration) Scintillation counter hodoscopes covering a solid angle of almost 4p R – 601: CERN – Rome collaboration Measurement of proton – proton elastic scattering at very small angles, including the Coulomb interference region
An original system of “pots” equipped with scintillator hodoscopes moving as close as possible to the beams under stable beam conditions The technique of movable “pots” (with different types of detectors) has since been used in all measurements of elastic and total cross-sections at all hadron colliders: the p p colliders at CERN and Fermilab, and the LHC (TOTEM experiment) R – 602 : Aachen – CERN – Harvard – Genova – Torino collaboration Measurement of proton – proton elastic scattering using two septum magnets (dipoles) Relation between elastic scattering at small angles and the total cross-section dσ π dσ π 2 A(θ*) d t k2 d* k2 A(q*) strong interaction scattering amplitude; t = –2k2 (1 – cosq*) (4 –momentum transfer)2 ; k : momentum in the centre-of-mass q* : scattering angle reference frame Extrapolation to t = 0 (q* = 0) gives |A(0)|2 = [ReA(0)]2 + [ImA(0)]2 4π “Optical” theorem: σ Im A(0) TOT k
ReA The measurement of ds/d|t| in the Coulomb interference region gives = ImA
π dσ σ 4 TOT 1+ρ2 d t to The proton – proton total cross – section at the end of 1973
M. Holder et al. (Aachen – CERN – Harvard – Genova – Torino coll.), Phys. Lett. B36 (1971) 400 U. Amaldi et al. (CERN – Rome coll.), Phys. Lett. B43 (1973) 231; Phys. Lett. B44 (1973) 112 R. Amendolia et al. (Pisa – Stony Brook coll.), Phys. Lett. B44 (1973) 119 The invention of the hadronic calorimeter Searches for free quarks Searches for free quarks with fractional electric charge started in 1964, as soon as the quark model of hadrons based on SU(3) symmetry was proposed The main detector methods: . Low ionization density (for relativistic free quarks expect dE/dx between
1/9 and 4/9 of (dE/dx)MIP (MIP = Minimum Ionizing Particle) . Measured momentum (assuming |charge| = e) higher than average beam momentum Searches for free quarks at the CERN PS:
. Exposure of the 80 cm H2 bubble chamber to a 20 GeV beam (average bubble density for 1 MIP = 25 bubbles / cm) . Exposure of a heavy liquid (Freon) bubble chamber to a 16 GeV beam (average bubble density for 1 MIP = 20 bubbles / cm) Observe no track consistent with fractional electric charge particles Assuming that free quark production mechanism is dominated by p + nucleon p + nucleon + q + q , obtain typical upper limits of the order of < 1 free quark produced in 108 p – nucleon collisions The most sensitive search for free quarks at the PS: a counter experiment in a beam from an internal target J.V. Allaby et al., Nuovo Cim. A64 (1969) 75 A secondary beam from a PS internal target bombarded by 27 GeV protons T : trigger counters PH: scintillators for pulse height measurement SC: streamer chamber (isotropic spark chamber) Selected beam momentum : 32.6 GeV/c for |charge| = 1/3 (“super momentum”) 22 GeV/c for |charge| = 2/3
Assuming p + nucleon p + nucleon + q + q : < 1 particle / 1011 p –nucleon collisions for charge = –1/3 < 1 particle / 2x1010 p –nucleon collisions for charge = –2/3 Search for free quarks at the ISR CERN – MPI coll., Nucl. Phys. B101 (1975)349
Assuming p + p p + p + q + q : < 1 particle / 109 p –p collisions for |charge| = 1/3 < 1 particle / 5x108 p –p collisions for |charge| = 2/3 at a collision energy of 53 GeV The three “g – 2” experiments Measurement of the muon anomalous magnetic moment For a review see F.J.M. Farley and E. Picasso, Ann. Rev. Nucl. Part. Science 29 (1979) 243 e Muon magnetic moment gm 2mc 2 For a spin ½ particle obeying the Dirac equation g = 2 Quantum fluctuations of the electromagnetic field around the muon modify this value:
gm -2 gm 2(1+ am ) am 2
Anomalous magnetic moment am ≈ 1 / 850
Original motivation to measure am in the early 1960s: to understand the difference between muon and electron (mass difference associated with different interaction?) Inject longitudinally polarized muons with momentum p into a uniform magnetic field B e B muon angular velocity ( / 2p “cyclotron frequency”) c - c mc e 1 muon spin precession s - + am B mc e a s -c - am B spin precession relative to the momentum vector mc An independent, precise measurement of the muon spin precession at rest provides the value of gm (e / mc) – B is measured precisely using proton magnetic resonance
am = 0 a > 0 m momentum and spin B spin turns faster are always aligned to figure than momentum
One full momentum turn angle between spin and momentum 2pam ≈ /135 need many turns to measure am precisely 1962 – 65: First CERN experiment with slow muons from the 600 MeV syncrocyclotron Special dipole magnet with gradients for muon focusing and orbit horizontal movement Measure muon polarization after 440 turns
Time modulation from am ≠ 0 vs. storage time
-3 am (1.162 0.005)10 1966 – 70: 1st muon storage ring at CERN (orbit diameter 5 m) p = 1.28 GeV/c, = 12, measure muon polarization over 2500 turns
Weak focusing magnetic field B = 1.711 T 10.5 GeV/c protons from the CERN PS (10 ns pulses) Time modulation from am ≠ 0 vs. storage time
-3 am (1.16616 0.00031)10 Storage rings require focusing to keep circulating beam inside vacuum chamber. This is usually achieved using magnetic field gradients (quadrupole components). In the 1st muon storage ring at CERN DB/B ≈ 0.2% over the full radial aperture of 8 cm the knowledge of the radial distribution of the circulating muons introduces
a systematic uncertainty on the measurement of am New idea: use uniform magnetic field and electrostatic focusing e 1 In the presence of an electrostatic field E - a B + - a b E a m 2 m (b = muon v/c ) mc -1 Idea first implemented in the 2nd muon storage ring at CERN factor = 0 (1972 – 76 , bending radius 7 m) for = 29.304 p = 3.094 GeV/c “magic” momentum Time modulation from am ≠ 0 vs. storage time
-3 am (1.165924 0.000009)10 1997 – 2006: The muon storage ring at Brookhaven National Laboratories (BNL): continuous superconducting magnet, bending radius 7 m, “magic” momentum muons -3 am (1.1659209 0.0000006 )10 CONCLUSIONS
The way to do particle physics experiments has changed a lot during the years 1964 – 74. . Improvements of detector technology: − from small volume to large volume bubble chambers; − from spark chambers to MWPCs and drift chambers; − development of calorimetry; − development of fast electronics and computers. . More theoretical guidance when proposing and designing new experiments, thanks to a better understanding of the laws of Nature (e.g., hadron compositeness and the development of the Standard Model)